The present invention relates to a video signal processing apparatus having the function of picture quality control for automatically adjusting the picture quality in a proper manner, and a signal processing circuit using the same system.
The home-use VTR is cited as an example of a conventional system relating to the present invention, as disclosed in "Video Technology Handbook" published on Nov. 20 in 1988, pp. 73 to 87, compiled by Editorial Dept. of Audio Video, a monthly Publication. This conventional apparatus will be described below with reference to FIG. 18. For simplification, reference will be made to a VHS system corresponding to the NTSC signal.
FIG. 18 is a block diagram showing an example of a conventional reproduction system for the home-use VTR. Prior to dealing with this diagram, a video signal to be recorded in a magnetic tape 1 will be described. The video signal is separated into a video luminance signal and a color signal, and the video luminance signal is subjected to FM modulation. In the process of FM modulation, the modulated carrier frequency is high when the luminance of a picture as a whole is high (with a bright screen), and low when the luminance is low (with a dark screen). This carrier frequency changes in dependency on the image brightness within a range of 3.4 to 4.4 MHz. On the other hand, the color signal, which is originally an amplitude and phase-modulated signal with a carrier of frequency 3.58 MHz, is changed to a carrier frequency of 629 kHz. The color signal and the luminance signal are thus recorded in separate frequency bands of the magnetic tape 1, the former in a low frequency range and the latter in a high frequency range.
In reproducing the signal thus recorded, in FIG. 18, the FM luminance signal and the low-frequency color signal recorded in the magnetic tape 1 are detected by a magnetic head 2, amplified by a reproduction amplifier 3, and further processed by an FM peaking circuit 4. Peaking processing of the FM peaking circuit 4 recovers the balance between the carrier frequency component and the side band components which are lost due to attenuation of high-frequency components in the magnetic recording and reproducing process of the FM luminance signal. So, the peak frequency of the FM peaking circuit 4 is set in its characteristic which is almost equal to the FM carrier frequency.
At the next step, the signal thus processed is applied to a low-pass filter (hereinafter referred to as "LPF") 5 and a high-pass filter (hereinafter referred to as "HPF") 8. The LPF 5 selectively passes only a low-frequency color signal spreading around 629 kHz. This signal is changed by a frequency converter 6 and a bandpass filter (hereinafter referred to as "BPF") 7 to a high-frequency color signal of around 3.58 MHz, and then applied to a mixer 12. The HPF 8, on the other hand, selectively passes the FM luminance signal with a carrier frequency set between 3.4 MHz and 4.4 MHz, and the resulting signal, after being demodulated by an FM demodulator 9 and the LPF 10 into a video luminance signal, is applied to a picture quality control circuit 11. The picture quality control circuit 11 has the function of reducing the noise and enhancing the contour of a reproduced image by changing the gain-to-frequency characteristic of the signal applied thereto. The output of the picture quality control circuit 11 is applied to the mixer 12, and after being mixed to a high-frequency color signal, is output as a reproduction video signal from a terminal 13. The picture quality control circuit 11 normally changes in characteristic by voltage control. The picture quality control circuit 11 is conventionally used for reducing the noise of a reproduced picture or enhancing the contour information by the user manually controlling a variable resistor or the like as desired.
According to the prior art described above, in view of the fact that the picture quality associated with a reproduced signal is manually controlled by the user, the condition for control is required to be readjusted to an optimum point in accordance with the change in signal-to-noise ratio (hereinafter referred to as "S/N") attributable to the tape deterioration each time the reproduction tape is changed, if a superior picture quality is to be enjoyed. This involves a very complicated picture quality adjusting operation. Also, even during the continuous reproduction of the same tape, the different noticeabilities of noise depending on the brightness of the reproduced screen differentiates the condition for an optimum picture quality for each occasion. It is very difficult to finely adjust the condition for control manually in accordance with such differences.
The same inventors invented a video signal processing apparatus for automatically controlling the picture quality of a reproduced video signal, and filed a U.S. patent application under U.S. Ser. No. 07/826,621 on Jan. 28, 1992. It has been found, however, that an always uniform automatic picture quality control for all reproduction modes including SP mode, EP mode, dubbing mode and still picture mode does not necessarily reproduce a superior picture of a video signal.